Spatially resolved equalization and forward error correction for multimode fiber links

ABSTRACT

A system is described that includes a system for correcting modal dispersion and errors in an optical fiber system. The system includes a multisegment photodetector coupled to an end of an optical fiber for detecting optical signals exiting the optical fiber and for converting the optical signals to an electrical output, the multisegment photodetector including a plurality of photodetector regions configured such that one of the plurality of photodetectors regions intercepts a mode in a manner distinct from another of the plurality of photodetectors. The system also includes logic configured to receive a resultant signal output from the photodetector regions and provide forward error correction decoding of the resultant signal.

TECHNICAL FIELD

The present invention is generally related to optical fiber systems and,more particularly, is related to signal processing in multimode fiberlinks.

BACKGROUND OF THE INVENTION

In an optical communication system deploying a multi-mode fiber, anoptical signal launched into the fiber propagates along the fiber asmultiple modes, each of which exhibits a different group velocity. Aportion of the source optical signal resides in the different modes. Themultiple modes can have different arrival times at the end of the fiber.The different group velocities of the modes cause a pulse formed frommore than one mode to spread out as it propagates, and is referred to asintermodal dispersion, which distorts the optical signal. Intermodaldispersion causes the optical signal initially launched through a fiberat a predetermined frequency and an initial phase to vary as a functionof the length of the fiber.

Modal dispersion reduces the maximum data transmission rate of theoptical communication system and thus diminishes the total transmissioncapacity of the fiber. This results at least in part from the fact thatmodal dispersion spreads the optical pulse as it propagates. Thus, shortpulses are limited to very short transmission distances and longerpulses can be transmitted further since the relative distortion of thepulse is smaller. Since shorter pulses typically require more bandwidth,multimode fiber is characterized by a bandwidth-distance product.Importantly, the bandwidth-distance products of typical multimode fiberare severely limiting. Modern multimode fiber incorporates a gradedoptical index profile within the core of the fiber to reduce modaldispersion. Unfortunately, modal dispersion remains the dominantbandwidth limiting mechanism in multimode fibers. Furthermore, ascompared to single mode fibers, these limits reduce the capacity ofmultimode fiber by orders of magnitude.

The different propagation velocities of the distinct optical modes canlead to large differential mode delay (DMD) in multimode fibers. Theseverity of this modal dispersion can limit multimode fiber links tobandwidth-distance products of a few 500 MHz-kilometers. DMD results inintersymbol interference (ISI).

Thus, a need exists in the industry to address the aforementioned and/orother deficiencies and/or inadequacies.

SUMMARY OF THE INVENTION

The present invention includes, among others, systems and methods thatcorrect for modal dispersion and errors in an optical fiber system.Briefly described, one embodiment of the invention can be implemented asa system that includes a multisegment photodetector coupled to an end ofan optical fiber for detecting optical signals exiting the optical fiberand for converting the optical signals to an electrical output, themultisegment photodetector including a plurality of photodetectorregions configured such that one of the plurality of photodetectorregions intercepts a mode in a manner distinct from another of theplurality of photodetectors. The system also includes logic configuredto receive a resultant signal output from the photodetector regions andprovide forward error correction decoding of the resultant signal.

The present invention can also be described as a method that, in oneembodiment, includes the following steps: detecting a plurality ofoptical signals radiating from an end of the multi-mode fiber by amultisegment photodetector having different detector regions that detectdifferent portions of the plurality of optical signals; modifyingdetected signals by the multisegment photodetector to reduce effects ofmodal dispersion among the plurality of optical signals; and forwarderror correcting the modified detected signals. Other systems, methods,features, and advantages of the present invention will be or may becomeapparent to one with skill in the art upon examination of the followingdrawings and detailed description. It is intended that all suchadditional systems, methods, features, and advantages be included withinthis description, be within the scope of the present invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference tothe following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a block diagram of an example optical fiber communicationsystem including an example detection, compensation, and correction(DCC) system that includes an example photodetection system and anexample forward error correction (FEC) decoding system, in accordancewith one embodiment of the invention.

FIG. 2 is a timing diagram depicting functionality or operation of theexample DCC system depicted in FIG. 1, in accordance with one embodimentof the invention.

FIG. 3 is a schematic diagram of one embodiment of an examplephotodetection system for use with the example optical fibercommunication system of FIG. 1.

FIG. 4 is a schematic diagram of an alternative embodiment of an examplephotodetection system for use with the example optical fibercommunication system of FIG. 1.

FIG. 5 is a schematic diagram of one embodiment of an example signalmodifier used to modify the signals from the example photodetector ofthe example photodetection systems shown in FIG. 3 and/or FIG. 4.

FIG. 6 is a functional block diagram that illustrates one exampledecoder embodiment of the example FEC decoding system depicted in FIG. 1for decoding product codes, in accordance with one embodiment of theinvention.

FIG. 7A is a schematic diagram of select internal circuitry of theexample FEC decoding system depicted in FIG. 1, in accordance with oneembodiment of the invention.

FIG. 7B is a schematic diagram of select internal circuitry of theexample FEC decoding system depicted in FIG. 1, in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the invention now will be described morefully hereinafter with reference to the accompanying drawings. One wayof understanding the preferred embodiments of the invention includesviewing them within the context of an optical fiber communicationsystem, and more particularly within the context of a system thatincludes functionality for photodetection, modal dispersioncompensation, and/or error correction of a signal carrying informationthat is transferred over a multimode optical fiber communication system.This system, herein referred to as the detection-compensation-correction(DCC) system, preferably implements spatial resolution and equalizationfor detection and compensation of modal dispersion. The spatialproperties of each mode (e.g., channels) delivered over the multimodefiber of the optical fiber communication system results in a spatialdiversity of the temporal response within the emitted optical spot.

In one embodiment, a multisegmented photodetector of the DCC system isused to exploit this spatial diversity (i.e., spatial resolution) toimprove the bandwidth of the optical fiber communication system.Specifically, concentric, multi-segmented photodetectors are used toreceive the optical modes, and the output of these segments are weightedand recombined (i.e., equalization). The magnitude and/or the phase ofthe signals output from the segments can be modified such that theresulting signals have the optimum temporal response. For example, theequalization can be limited with a fixed scalar weighting.

The DCC system also implements various forward error correction (FEC)methods to decode the spatially resolved and equalized signal. Herein,decoding will be understood to include error detection and/or errorcorrection functionality. As indicated above, the temporal response ofthe optical fiber communication system can be severely affected bydifferential mode delay (DMD). The spatial resolution and equalizationmechanisms alluded to above can eliminate some of the interference, buttypically at the expense of some power loss at the photodetector output.The coding gain of an FEC decoding system of the DCC system will be usedto “recover” some of this power loss. The coding gain is generallydefined as the difference between the E_(b)/N₀ ratio needed to achieve agiven bit error rate probability with coding and without coding, whereE_(b) is the energy per information bit and N₀ is the (one-sided) noisepower spectral density, as is well known to those having ordinary skillin the art. In other words, by not implementing any error correctioncoding, the coding gain answers the question as to how much of a powerincrease is needed to get the same bit error rate. By “recovering” thepower loss, the DCC system is not necessarily adding power, but becauseof the coding gain, the data rate can be increased considerably withoutan increased bit error rate (BER).

Because the preferred embodiments of the invention can be understood inthe context of an optical fiber communication system, an initial generaldescription of a multimode optical fiber communication system isprovided that illustrates, among other components, the DCC system.Following this initial general description is a timing diagram thatillustrates how the DCC system processes optical signals received fromthe optical fiber communication system. The description of the timingdiagram is followed by two embodiments of a photodetection system, andfurther followed by an embodiment of a signal modifier for use with oneor more embodiments of the photodetection system. One embodiment of theFEC decoding system is then described in the context of iterative softdecision decoding of product codes, followed by two example embodimentsof the FEC decoding system for this latter implementation.

The FEC decoding system of the DCC system can operate on a plurality ofFEC coding formats, including block codes (e.g., Reed-Solomon (RS)codes, BCH codes, etc.), product codes (e.g., two-dimensional blockcodes), convolutional codes, turbo codes, low density parity-check(LDPC) codes, among others. The FEC decoding system will be described inthe context of an implementation that operates on product codes as oneexample implementation among others. The product codes will be describedherein using a matrix format (e.g., rows and columns of symbols), withthe understanding that product codes will not be limited to this matrixformat but can take the form of substantially any encoded format usedfor transmitting data, whether formatted in ordered and/or randomfashion. Generally, the product codes described herein will preferablyinclude those formats exhibiting characteristics that include some formof error correction or control code iteration, some mechanism forgathering extrinsic information (e.g., information that can be used todetermine the reliability of one or more symbol values), and some formof diversity (e.g., independence in row and column decoding operations).Note that extrinsic information will herein be understood to includereal numbered values received from the photodetection system in additionto reliability information passed between a row and column decoder (orshared between row and column decoding operations).

The preferred embodiments of the invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those having ordinary skill in theart. Furthermore, all “examples” given herein are intended to benon-limiting, and are provided as an exemplary list among other examplescontemplated but not shown.

Referring now in more detail to the drawings, in which like numeralsindicate corresponding parts throughout the several views, FIG. 1 is aschematic diagram of an optical fiber communication system 100 thatincludes an FEC Encoding System 110, an electrical-to-optical (EO)transmitter system 120, a multimode fiber 130, and a DCC system 140 thatincludes a photodetection system 142 and an FEC decoding system 145. TheFEC encoding system 110 preferably receives data from a signal source(not shown), such as a computer or a consumer electronics device, amongother electrical and/or mechanical devices. Although shown as a separateentity, the FEC encoding system 110 can be integrated with the signalsource in some embodiments. The FEC encoding system preferably includesencoders 111 and 112.

As indicated above, the information can be encoded according tosubstantially any FEC scheme. As one example implementation among many,an FEC scheme is described below implementing product codes in a matrixformat of rows and columns. Product codes in a matrix format can berepresented mathematically. In general, a two dimensional product codeis obtained as follows: the information symbols are initially arrangedin a k₁×k₂ array. Then, the columns are encoded using a linear blockcode C₁ (n₁, k₁, λ₁). Afterwards, the resulting n₁ rows are encodedusing a linear block code C₂ (n₂, k₂, λ₂) and the product code, whichconsists of n_(i) rows and n₂ columns is obtained. The parameters ofcode C_(i) (i=1,2), denoted as n_(i), k_(i), and λ₁, are the codewordlength, number of information symbols, and minimum Hamming distance,respectively. Codes C₁ and C₂ are called the constituent (or component)codes. The parameters of the resultant product code are n_(C)=n₁n₂,k_(C)=k₁k₂, λ_(C)=λ₁λ₂, and the code rate is R_(C)=R₁R₂, whereR_(i)=k₁/n_(i). To decrease implementation complexity of the encoders111 and 112 and the decoder 148, preferably the same block code isselected as the row and column constituent code (i.e., C₁=C₂).

Generally in a product code FEC scheme, information is encoded atencoder 111 with a first level of error correction information (e.g.,parity). This information and parity can be ordered into a definedformat, or in other embodiments, preferably randomized at encoder 111and then passed to a second encoder 112 where it is encoded with anotherlevel of parity, and then output to the EO transmitter system 120 (i.e.,data is sent to the EO transmitter 120 preferably in a serial fashion.For example, symbols (e.g., bits) are read out row-by-row, orcolumn-by-column. At the FEC decoding side, explained below, the decoder148 re-orders the data into the matrix form).

The EO transmitter system 120 preferably converts the encoded electronicsignal to an optical signal, the process of which may include the use ofa digital-to-analog converter 122 and a light source, such as a laser124. The laser 124 launches an optical signal into a multimode fiber130. The optical signal propagates along the multimode fiber 130. As theoptical signal propagates in the multimode fiber 130, multiple modes oflight are formed with many travelling through the multimode fiber 130 atdifferent velocities. Preferably, wavelengths in which the laser 124operates includes 850 nanometers (nm) and/or 1300 nm, among otherpossible wavelengths. A suitable laser includes a vertical cavitysurface emitting laser (VCSEL) that is well known in the art, but mayalso include light emitting diodes (LED), distributed feedback (DFB)lasers or Fabry-Perot (F-P) lasers.

Positioned at the end of the multimode fiber 130 is the DCC system 140.In one implementation, multiple modes of light exiting from themultimode fiber 130 enter the photodetection system 142. Thephotodetection system 142 includes a photodetector, such asmulti-segmented photodetector 143, for detection and compensation ofmodal dispersion. The photodetection system 142 can also include asignal modifier 580 in alternate embodiments (as represented by thedotted line box boundary) that compensates for the modal dispersion.

At the photodetector 143, the plurality of detected signals are alteredand combined into one electrical output. This function occurs via theuse of varying biases to alter (relative to each other) the individualelectrical signal of each multiple segment or in cooperation with theseparate signal modifier 580 in alternate embodiments. The photodetector143 outputs an electrical signal that, if converted to an opticalsignal, is substantially similar to the originally transmitted opticalsignal launched from the laser 124 into the multimode fiber 130. It ispossible to implement any necessary alteration, or weighting factor, tothe plurality of detected signals by the use of appropriate bias amongthe plurality of detection segments. However, the signal modifier 580may be used in alternate embodiments to adjust the signal as necessary.The selection of weighting factors is preferably chosen to reducedifferences between the various modes. Moreover, weighting factors canbe fixed or adjustable. The signal modifier 580 can include electronicand/or mechanical devices such as attenuators, delay lines, amplifiers,and/or phase shifters.

The spatially resolved and equalized output signal of the photodetectionsystem 142 is then input to the FEC decoding system 145. The FECdecoding system 145 includes functionality for decoding one dimensionalblock codes (such as Reed-Solomon or BCH codes), multi-dimensional blockcodes (e.g., product codes like Turbo product codes), convolutionalcodes, turbo codes, or LDPC codes. In one embodiment, the FEC decodingsystem 145 includes a decoder 148 and a threshold detector 146. Althoughshown as separate components, functionality of each component can bemerged into a single component in some embodiments. In some embodiments,threshold detector functionality can be implemented externally to theFEC decoding system 145, as explained below. The decoder 148 preferablyincludes functionality for sequential or, in some embodiments,substantially simultaneous row and column decoding, in accordance withone embodiment of the invention. Further information on substantiallysimultaneous row and column decoding can be found in the commonlyassigned patent application entitled, “A Parallel Decoder For ProductCodes”, filed on the same date, assigned to Georgia Tech, and hereinincorporated by reference. In one example implementation, the spatiallyresolved and equalized information can be received as symbols (e.g.,data) formatted as voltage values from the spatial resolution andequalization (including weighting) at the photodetection system 142.

The decoder 148 preferably decodes the rows and columns of the productcodes. In cooperation with one or more threshold detectors, such asthreshold detector 146, the decoder 148 uses the real number valuedinformation of the received spatially resolved and equalized inputsignal to provide error correcting on the received signal, in accordancewith one embodiment of the invention. In one implementation, thethreshold detector 146 performs a comparator function where it comparesthe voltage values received at the decoder 148 to a defined thresholdvalue to provide the decoder 148 with an indication of the proximity ofthe voltage value to a decided binary value (as decided by the decoder148). In other implementations, the threshold detector 146 performs moreof a “threshold” function, where it receives the spatially resolved andequalized input signal and “thresholds” the received values to bit orbyte values, which the decoder 148 can perform decoding iterations on.

Preferably, the decoder 148 and the threshold detector 146 will operateusing a combination of real number values and byte and/or bit valuesduring the various stages of decoding. For example, the real numberedvoltage value or values that are received by the decoder 148 can beloaded into the threshold detector 146, which then returns bit valuesthat include values that are “flagged” as unreliable by the decoder 148.The decoder 148 can run error correcting iterations on the bits toprovide an update on the reliability of the bits, then use the thresholddetector 146 (or another threshold detector) to return the values toupdated real number values to pass on to a next decoding stage. Notethat other components, although not shown, can also be included in theDCC system 145, including memory, demodulators, analog to digitalconverters, processors, among others as would be understood by onehaving ordinary skill in the art.

FIG. 2 shows a timing diagram depicting components and componentinteractions encompassed by the DCC system 140 depicted in FIG. 1, inaccordance with one embodiment of the invention. At step 222, thephotodetector 143 of the photodetection system 142 receives the variouspropagating modes of an optical signal from the multimode fiber 130(FIG. 1). In one implementation, the photodetector 143 performs spatialresolution, biases the signals to alter the magnitude and/or polarity ofthe received signals, and then recombines them to provide a singleoutput voltage (step 224 a). In another embodiment (denoted throughsteps 224 b and 224 c), the photodetector 143 provides spatialresolution and then passes the signals to the signal modifier 580 (step224 b). The signal modifier 580 performs a weighting and summationoperation, and passes the signal to the threshold detector 146 (step 224c) of the FEC decoding system 145. The decoder 148 preferably determinesthe reliability of the received signal using the real number valuedinformation it received from the photodetection system 142 and throughcooperation with the threshold detector 146 (step 226).

For example, a received value can have a value of 0.1 V (assuming asystem where a binary 1 value is 5.0V and a binary 0 value is 0.0V).Assume a threshold value configured at the threshold detector 146 of2.5V. The decoder 148 recognizes that the 0.1V is very close to zero.Receiving input from the threshold detector 146 (or polling thethreshold detector 146), the decoder 148 determines that the firstvoltage value is also much less than the threshold value. Voltage valueswith a high degree of reliability will preferably be updated to make thevalues more reliable and then they are passed to the column decoder (notshown) of the decoder 148 where further error correcting is employed,preferably independent of the row decoding. Voltage values with a lowdegree of reliability (e.g., near the threshold value) will also beupdated, and that updated value will be used in column decoding.

Continuing the example, assume a next received voltage value of 2.4V isrecognized by the decoder 148 as having a reliability issue, since 2.4V,although closer to 0.0V than 5.0 volts, is “near” the 2.5V voltagethreshold of the threshold detector 146, and thus this voltage value is“flagged” as having suspect reliability. The decoder 148 can proceed torun an algorithm, for example a well-known Chase algorithm or a variantthereof, to evaluate many different permutations of values for thisunreliable value (and others that follow) to determine an updated valuethat will be passed to the column decoder to improve (or reduce) thereliability of the voltage value for column decoding. Note that furtherinformation on some algorithms employed to update values can be found inthe references entitled, “Near-optimum decoding of product codes: Blockturbo codes,” IEEE Trans. Commun., vol. 46, no. 8, pp. 1003-1010, August1998, and D. Chase, “A class of algorithms for decoding block codes withchannel measurement information,” IEEE Trans. Inform. Theory, vol.IT-18, no. 1, pp. 170-182, January 1972, and the commonly assigned(assigned to Georgia Tech) patent application filed on the same date,and entitled, “Efficient Decoding of Product Codes”, all threereferences which are herein incorporated by reference.

If the decoder 148, through running various permutations, determines avalue that indicates the generated bit value should be zero and not one,then the row decoder of the decoder 148 preferably updates the value of2.4V with a new value. The new value is preferably more indicative of a0 bit value, say 2.0V for example, to provide a voltage value to thecolumn decoder of the decoder 148 that is more reliable. Note that thecolumn decoder may process this symbol position and determine that thevalue is more indicative of a binary “1”, and thus a “reliabilitytug-of-war” between row and column decoding can occur through severaliterations. Thus, even before the decoding process begins, the decoder148 has information about which symbols are unreliable and which symbolsare reliable, based on the voltage value received from thephotodetection system 142 and its proximity in value to the definedthreshold value configured at the threshold detector 146, or elsewhere.

Finally, the error corrected, decoded signal is preferably output by thedecoder 148 to a device, such as a computer or other device, or in otherembodiments, the decoder 148 is internal to a device that providesfurther processing of the decoded signal (step 228).

FIG. 3 is a schematic diagram of one embodiment of the examplephotodetection system 142 that could be used with the optical fibercommunication system 100 of FIG. 1. The example photodetection system142 includes the example photodetector 143. The example photodetector143 is preferably multi-segmented, and includes associated biasingcircuitry. The photodetector 143 receives a plurality of optical signals152 exiting the multi-mode fiber 130. The photodetector 143 may beshaped and arranged in a number of arbitrary manners. The examplephotodetector 143 includes coplanar, annular detector segments 154, 155,and 156. The photodetector 143 is not limited to having coplanar,annular photodetection segments, and other embodiments of thephotodetector 143 may include non-planar, non-annular and/ornon-concentric photodetector segments.

The photodetector 143 is not limited to the number of detector segmentsshown in the configuration of FIG. 3. Each individual detector of thephotodetector 143 detects the plurality of optical signals 152 (alsoreferred to as modes) differently according to each detector's positionrelative to the multi-mode fiber 130.

Also shown in FIG. 3 is one example mechanism for adjusting the signals152. The optical signals 152 carry a portion of the original opticalsignal launched in the fiber 130 by the laser 124 (FIG. 1) and a portionof interference optical modes caused by modal dispersion. Generally, thegeometric configuration of the detector segments 154, 155, and 156provides for isolating an optical signal 152, wherein the isolatedsignal is a combination of the original optical signal and interferencesignals generated from the multiple modes. The three segments receivebias 158, 160, and 162.

In one implementation, segment 156 detects a combination of modes havinga weak portion of the original optical signal and a portion of theinterference signal. This segment 156 is positively biased by 160,producing no change in the polarity of the mode. Segments 154 and 155detect a combination of modes having a strong portion of the originaloptical signal and a portion of the interference signal. These segmentsare negative biased by 158 and 162 causing the respective interferingsignal to have a negative polarity. Combining all these segment signalscauses a cancellation of the interference signals and produces an outputsignal that closely approximates the optical signal originally launchedinto the fiber 130.

Bias 158, 160, and 162, such as a voltage bias, applied across thecoplanar, annular segments 154, 155, and 156 of the photodetector 143modifies the signals produced from the photodetector 143 and assists inthe direction and flow of electrons producing an output, such as voltageV_(out) at contact 164. The biases 158, 160, and 162 shown in FIG. 3 arespecific examples of weighting factors. The signals are weighted basedon a different bias magnitude applied to the photodetector 143 and oneor more of their polarity is changed. When summed, the detected signalsresult in an output that can closely approximate the original opticalsignal coupled into the fiber 130.

In one embodiment, as described above, the modification made to theindividually detected signals is by applying a bias of differentmagnitude and/or polarity to each segment. In an alternative embodiment,the modification is achieved by any combination of electrical and/ormechanical instruments used to impart changes in amplitude and/or phaseto the electrical signal. Additionally, a number of diffractive orreflective optical elements may be positioned between the multi-modefiber 130 and the photodetector 143.

FIG. 4 shows a schematic diagram of an alternative embodiment of thephotodetection system 142 shown in FIG. 3, the alternative embodimentreferred to as photodetection system 442. The photodetection system 442includes a photodetector 443 that is preferably multi-segmented. Thephotodetector 443 receives a plurality of optical signals 152 from amultimode fiber 130. A number of diffractive or reflective opticalelements may be positioned between the multimode fiber 130 and thephotodetector 443. For instance, a diffractive element 468 between thephotodetector 443 and the multimode fiber 130 refocuses the opticalsignals 152 in a specific manner (e.g., the diffractive element 468 canredirect the mode to a specific segment) before the optical signals 152enter the photodetector 443. The intervening optical elements caninclude, but are not limited to, lenses, mirrors and/or holographicelements. The detector segments of the photodetector 443 receiveidentical biases, and thus an additional, external means of applying thedesired weighting factors is used for compensation for modal dispersion.

A direct current (DC) bias 470 across the external input contact 467 ofthe photodetector 443 provides an electrical force to cause thegenerated electrons to exit the segments through the appropriate contact(472, 474, or 476) defining the segment (e.g., through contact 472 ifthe electrons are generated between contacts 467 and 472, throughcontact 474 if generated between contacts 467 and 474, etc.). As shownin FIG. 4, the diffractive element 468 affects a change in direction ofthe output signals at contacts 472, 474, and 476. For illustrativepurposes only, three outputs at contacts 472, 474, and 476 exit thephotodetector 443. The photodetector 443 is not limited to producingonly three output signals.

FIG. 5 is a schematic diagram of an embodiment of a signal modifier 580used to modify the signals from the photodetectors 143, 443 of FIG. 3and/or FIG. 4. Specifically, this signal modifier 580 is a systemrepresentation of what is occurring to the signal from each segment. ForFIG. 3, the functionality of the signal modifier 580 occurs internally(i.e., via the bias circuit and multi-segment photodetector 143) throughproper selection of the bias. Signals V₁, V₂, V₃ to V_(n) (at contacts472, 474, 476, and 478, respectively) can be modified by weightingfactors as discussed above including incorporating bias, attenuation,amplification and/or delay. For illustrative purposes, signals at 472,474, 476, and 478 experience an arbitrary vector scaling factor 582,584, 586, and 588. This scaling factor can be provided by the biasselection (for the embodiment illustrated in FIG. 3) or via a discretecomponent not shown (in the embodiment illustrated in FIG. 4), as oneexample. The signals at 472, 474, 476, and 478 may be subjected toanother weighting factor or a combination of weighting factors. Thedetermination of the specific modification to be applied to eachdetected signal may be done in several ways including, but not limitedto, the use of fixed, arbitrary settings; the use of techniques duringwhich a known signal is transmitted and the modification is setsystematically and/or randomly until the detected output replicates theknown signal; or by the use of computing (analog and/or digital)software and/or hardware to apply adjustments to the detected signals tosatisfy any other criteria set by the user or designer of the system.

Interim outputs 590, 592, 594, and 596 are summed by summer 506 toproduce an output signal 508 that, if an equivalent optical signal,closely approximates the optical signal originally coupled into thefiber 130 (FIG. 1). The output signal 508 can be used in a variety ofmanners including converting the signal to digital using ananalog-to-digital converter (not shown) and then providing the signal toa device such as a computer for use in a user's application. Furtherinformation on the spatial resolution and equalization of a receivedoptical signal can be found in the commonly assigned (assigned toGeorgia Tech) patent applications entitled, “Compensation of ModalDispersion in Optical Waveguides”, filed Feb. 1, 2002, and “SegmentedPhotodetectors for Detection and Compensation of Modal Dispersion inOptical Waveguides”, filed Feb. 1, 2002, both incorporated herein byreference. Preferably, the output signal 508 is transferred to the FECdecoding system 145 (FIG. 1), as described below.

One goal of the FEC decoding system 145 (FIG. 1) is to provide codinggain to “recover” from the power loss that can arise from the spatialresolution and equalization process of the photodetection system 142(FIG. 1). As described above, the FEC decoding system 145 can implementerror correction on block codes formatted in one dimension or twodimensions. An example implementation will now be described for twodimensional product codes wherein iterative soft decision processing isimplemented, and wherein the voltage values (or current values) received(e.g., output signal 164 (FIG. 3) or output signal 508 (FIG. 5)) arereal number valued and thus used by the FEC decoding system 145 todetermine the reliability of the received values. It is noted, asindicated above, that the following implementation includes one exampleFEC mechanism, and that substantially all types of FEC codes (e.g.,turbo product codes, product/array codes, turbo codes, block codes,convolutional codes, LDPC codes, etc.) are included within the scope ofthe invention. The process of iterative soft decision decoding ofproduct codes can generally be described mathematically. The decoder 148(FIG. 1A) preferably applies a Chase algorithm, or variants thereof, orother decoding algorithms, iteratively on voltage values formatted inrows and columns. The Chase algorithm is a sub-optimum decoding methodbased on forming test patterns preferably using real number valueinformation from the photodetection system 142 (FIG. 1) and passingthese test patterns through an algebraic decoder for the employed blockcode. These test patterns are formed by perturbing the p least reliablesymbol positions (p being a fixed or variable design parameter that ispreferably experimentally determined) in the received noisy sequence (pis selected as p<<k). The number of test patterns is equal to 2^(p).After decoding of the test patterns, the most likely among the generatedcandidate codewords is assigned as the decided codeword.

The reliability information for symbol position j is expressed in termsof a log-likelihood ratio (LLR) given by:A(d _(j))=log [(Pr(c _(j)=+1|R))/(Pr(c _(j)=−1|R))],  (Eq. 1)where D=d₀ . . . d_(n−1)(d_(j)ε{−1, +1}) is the decided codeword afterChase decoding, R=r₀, . . . r_(n−)denotes the received noisy sequence,and C=c₀ . . . c_(n−1) is the transmitted codeword. If {circumflex over(D)}={circumflex over (d)} . . . {circumflex over (d)}_(n−1) (if itexists) is the most likely competing codeword among the candidatecodewords with {circumflex over (d)}_(j)≠d_(j), then for a stationaryadditive white Gaussian noise (AWGN) communication medium and acommunication system using binary phase shift keying (BPSK), forexample, the LLR of symbol position can be approximated by:A(d _(j))≈[(|R−{circumflex over (D)}| ² −|R−D| ²)/4]d _(j),  (Eq. 2)where |A−B|² denotes the squared Euclidean distance between vectors Aand B. Equation 2 is essentially describing how the new information(e.g., the updated voltage value) is computed. Note that othermechanisms for determining LLR can be employed, such as those usinginner product terms as described in the commonly assigned patentapplication entitled, “Efficient Decoding of Product Codes” asreferenced herein. The extrinsic information w_(j) for the jth position(e.g., the updated voltage values (symbol entries in the product codearrays) that are passed to the column decoder, and vice versa) is foundby:

-   -   w_(j)={A (d_(j))−r_(j), if a competing {circumflex over (D)}        exists, or βd_(j), if no competing {circumflex over (D)}},        (Eq. 3) where β is a reliability factor. Once the extrinsic        information has been determined, the input to the next decoding        stage is updated as        r _(j) ′=r _(j) +γw _(j),  (Eq. 4)        where γ is a weight factor introduced to combat high bit error        rate (BER) and high standard deviation in w_(j) during the first        iterations. In other words, for the example implementation        described above, the new r_(j) in a next stage decoding is the        old r_(j) (i.e., the initial voltage value received from the        photodetection system 142 (FIG. 1) (or various embodiments        herein) and entered at a position in the product code array)        plus a weighted updated voltage value. Preferably, γ is an        experimentally determined value. The operations above can be        performed on all symbols of a product codeword, hence equation 4        can be expressed as        [R′]=[R]+γ[W].  (Eq. 5)

A functional block diagram that illustrates the functionality of theexample decoder 148 (FIG. 1) at one stage is shown in FIG. 6 for aparallel decoding scheme, in accordance with one embodiment of theinvention. In other embodiments, a serial or sequential mechanism can beemployed for this information sharing, and other FEC decoding schemescan be employed by the decoder 148 as described above. As shown, theprior equations are somewhat reflected in the schematic. The exampledecoder 148 implemented for iterative soft decision decoding preferablyincludes at least one row decoder 640 and at least one column decoder642. The row and column decoders 640 and 642 operate here in paralleland update each other immediately after a row or column has beendecoded. Furthermore, a delay element 646 is utilized. Relating theabove math formulations to the example decoder of FIG. 6, the integer“m” denotes the number of the current full iteration. The [R] representsthat the extrinsic information (e.g., real number valued voltage) isused for each decoding and for each iteration. [R^(row)] and [R^(col)]in the first iteration are equal to [R], but once updated, the [R^(row)]includes the updated extrinsic information received from the columndecoder (y×[W^(col)]) and the [R^(col)] includes the updated extrinsicinformation received from the row (γ×[W^(row)]) to be passed to the nextiteration, along with the [R] channel information. The matrices[W^(row)] and [W^(col)] are the row and column extrinsic informationmatrices, respectively, and are transferred on a row—row orcolumn—column basis.

The m^(th) iteration of the row decoding process generates extrinsicinformation (e.g., updated voltage values) represented by [W^(row)].Substantially simultaneously, the column decoder 642 generates, on them^(th) iteration, extrinsic information [W^(col)]. These matrices arepreferably scaled by γ, which is preferably a function of the iteration,producing a scaling effect that increases, as the iterations continue,to one. Following the scaling, (γ×W), the scaled value is added to theoriginal received values R (e.g., the values received from thephotodetection system 142 (FIG. 1)) which gets fed to the respectivecolumn 642 or row decoder 640. Once this occurs for an entire productcode array, the new R (and old R) is passed on to the block again forthe m^(th)+1 iteration (i.e., the next iteration).

In one implementation, the m^(th)+1 iteration is fed back to the row andcolumn decoder 640 and 642. In other implementations, the block diagramof FIG. 6 is just replicated for the m^(th)+1 iteration. Because thefirst iteration for decoding the rows and columns takes a finite amountof time, there is a delay between iterations, as depicted by the delayelement 646. This is in contrast to the delays experienced in sequentialdecoding, which typically include a first delay until the firstiteration of the row decoding has occurred, and then a second delayuntil the first iteration of the column decoding has occurred. Note thatβ is a reliability factor, as explained above, which is essentially ascaling factor that handles circumstances where the row and/or columndecoder cannot implement decoding.

FIGS. 7A-7B are block diagram illustrations of select components of theFEC decoding system 145 of FIG. 1 in accordance with two embodiments ofthe invention. FIG. 7A illustrates the FEC decoding system 145A in whichthe decoder 148 is implemented as hardware, in accordance with oneembodiment. The decoder 148 can be custom made or a commerciallyavailable application specific integrated circuit (ASIC), for example,running embedded decoding software alone or in combination with themicroprocessor 158. That is, the decoding functionality can be includedin an ASIC that comprises, for example, a processing component such asan arithmetic logic unit for handling computations during the decodingof rows and columns. Data transfers to and from memory 159 and/or to andfrom the threshold detector 146 for the various matrices (as explainedbelow) during decoding can occur through direct memory access or viacooperation with the microprocessor 158, among other mechanisms. Themicroprocessor 158 is a hardware device for executing software,particularly that stored in memory 159. The microprocessor 158 can beany custom made or commercially available processor, a centralprocessing unit (CPU), an auxiliary processor among several processorsassociated with the decoder 148, a semiconductor based microprocessor(in the form of a microchip or chip set), a microprocessor, or generallyany device for executing software instructions. The threshold detector146 can be software and/or hardware that is a separate component in theFEC decoding system 145A, or in other embodiments, integrated with thedecoder 148, or still in other embodiments, omitted from the FECdecoding system 145 and implemented as an entity separate from the FECdecoding system 145 yet in communication with the FEC decoding system145. The -FEC decoding system 145 can include more components or canomit some of the elements shown, in some embodiments.

In one preferred embodiment, where the decoder 148 is implemented inhardware, the decoder 148 can be implemented with any or a combinationof the following technologies, which are each well known in the art: adiscrete logic circuit(s) having logic gates for implementing logicfunctions upon data signals, an ASIC having appropriate combinationallogic gates, a programmable gate array(s) (PGA), a field programmablegate array (FPGA), etc.

FIG. 7B describes another embodiment, wherein decoding software 160 isembodied as a programming structure in memory 169, as will be describedbelow. The memory 169 can include any one or combination of volatilememory elements (e.g., random access memory (RAM, such as DRAM, SRAM,SDRAM, etc.)) and nonvolatile memory elements (e.g., ROM, bard drive,tape, CDROM, etc.). Moreover, the memory 169 may incorporate electronic,magnetic, optical, and/or other types of storage media. Note that thememory 169 can have a distributed architecture, where various componentsare situated remote from one another, but can be accessed by themicroprocessor 168.

In one implementation, the software in memory 169 can include decodingsoftware 160, which provides executable instructions for implementingthe FEC decoding operations. The software in memory 169 may also includeone or more separate programs, each of which comprises an orderedlisting of executable instructions for implementing logical functionsand operating system functions such as controlling the execution ofother computer programs, providing scheduling, input-output control,file and data management, memory management, and communication controland related services.

When the FEC decoding system 145 (145A or 145B) is in operation, themicroprocessor 158 (or 168) is configured to execute software storedwithin the memory 159 (or 169) to communicate data to and from thememory 159 (or 169), and to generally control operations of the FECdecoding system 145A, 145B pursuant to the software.

When the decoding functionality is implemented in software, it should benoted that the decoding software 160 can be stored on anycomputer-readable medium for use by or in connection with any computerrelated system or method. In the context of this document, a computerreadable medium is an electronic, magnetic, optical, or other physicaldevice or means that can contain or store a computer program for use byor in connection with a computer related system or method.

The decoding software 160 and/or the decoder 148 can be embodied in anycomputer-readable medium for use by or in connection with an instructionexecution system, apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. In the context of this document, a“computer-readable medium” can be any means that can store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, or device. The computerreadable medium can be, for example but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, device, or propagation medium.

More specific examples (a nonexhaustive list) of the computer-readablemedium would include the following: an electrical connection(electronic) having one or more wires, a portable computer diskette(magnetic), a random access memory (RAM) (electronic), a read-onlymemory (ROM) (electronic), an erasable programmable read-only memory(EPROM, EEPROM, or Flash memory) (electronic), an optical fiber(optical), and a portable compact disc read-only memory (CDROM)(optical). Note that the computer-readable medium could even be paper oranother suitable medium upon which the program is printed, as theprogram can be electronically captured, via for instance opticalscanning of the paper or other medium, then compiled, interpreted orotherwise processed in a suitable manner if necessary, and then storedin a computer memory. In addition, the scope of the present inventionincludes embodying the functionality of the preferred embodiments of thepresent invention in logic embodied in hardware and/or softwareconfigured mediums.

It should be emphasized that the above-described embodiments of thepresent invention, particularly, any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the invention. Many variations andmodifications may be made to the above-described embodiment(s) of theinvention without departing substantially from the spirit and principlesof the invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and the presentinvention and protected by the following claims.

1. A system for correcting modal dispersion and errors in an opticalfiber system, comprising: a plurality of detection zones for detecting aplurality of modes of light incident on the plurality of detectionzones, the plurality of detection zones positioned on a substrate andarranged in a coplanar, annular configuration; a plurality of segmentslocated within the detection zones, each of the segments being adaptedto detect the plurality of modes; and logic configured to receive aresultant signal output from the detection zones and provide forwarderror correction decoding of the resultant signal.
 2. The system ofclaim 1, wherein the plurality of segments further comprisesinterdigitating, planar metal-semiconductor-metal (MSM).
 3. The systemof claim 1, wherein the plurality of detection zones are concentric,annular detection zones.
 4. The system of claim 1, wherein the pluralityof detection zones are coplanar detection zones.
 5. The system of claim1, wherein one detection zone detects a combination of modes that issubstantially distinct from a mode of light detected by other detectionzones.
 6. The system of claim 1, further comprising an optical elementplaced between the photodetector and a fiber for enhancing theseparation of a plurality of modes by the plurality of detection zones.7. The system of claim 6, wherein the optical element comprises adiffractive element.
 8. The system of claim 6, wherein the opticalelement comprises a binary diffractive element.
 9. The system of claim6, wherein the optical element comprises a holographic element.
 10. Thesystem of claim 1, wherein the logic is further configured to provideforward error correction for coding formats that include at least one ofblock codes, Reed-Solomon codes, product codes, turbo product codes,turbo codes, low density parity-check codes, and convolutional codes.11. The system of claim 1, wherein the logic is further configured todetermine the reliability of row symbols and column symbols for adecoded row and a decoded column based in part on real number valuedinformation received from the resultant output signal, wherein the logicis further configured to pass the reliability determinations of the rowsymbols to decode a next column of symbols and pass the reliabilitydeterminations of the column symbols to decode a next row of symbols.12. The system of claim 1, wherein the logic is included in at least oneof a discrete logic circuit having logic gates for implementing logicfunctions upon data signals, an application specific integrated circuithaving combinational logic gates, a programmable gate array, and a fieldprogrammable gate array.
 13. The system of claim 1, wherein the logic isincluded in a computer-readable medium.
 14. The system of claim 1,further including at least one of a processor, memory, and a thresholddevice that communicates with the logic in providing decodingfunctionality.
 15. The system of claim 14, wherein the processor and thelogic are located in separate devices.
 16. The system of claim 14,wherein the processor and the logic are located in the same device. 17.A system for correcting modal dispersion and errors in an optical fibersystem, comprising: a multisegment photodetector coupled to an end of anoptical fiber for detecting optical signals exiting the optical fiberand for converting the optical signals to an electrical output, themultisegment photodetector including a plurality of photodetectorregions configured such that one of the plurality of photodetectorsregions intercepts a mode in a manner distinct from another of theplurality of photodetectors; and logic configured to receive a resultantsignal output from the photodetector regions and provide forward errorcorrection decoding of the resultant signal.
 18. The system of claim 17,wherein the plurality of photodetector regions comprise an array ofcoplanar, annular regions.
 19. The system of claim 17, wherein theplurality of photodetector regions further comprises coplanar, circularsections having a plurality of interdigitated, planar MSM segments. 20.The system of claim 17, wherein the plurality of photodetector regionsfurther comprises a plurality of interdigitated segments representing aconductive portion of the plurality of photodetector regions.
 21. Thesystem of claim 17, wherein the plurality of photodetector regionsfurther comprises a plurality of doped semiconductor materials creatinga PIN structure.
 22. The system of claim 17, further comprising adiffractive element coupled between the multisegment photodetector andthe end of the optical fiber for directing the optical signals into themultisegment photodetector.
 23. The system of claim 17, furthercomprising an output circuit coupled to the plurality of photodetectorregions for modifying signals from the plurality of photodetectorregions and producing a signal substantially similar to an opticalsignal coupled into the optical fiber.
 24. The system of claim 17,further comprising an optical signal launched into the optical fiber.25. The system of claim 17, further comprising a network including atleast one of attenuators, amplifier, phase shifters and transmissionlines that modify a plurality of detected signals, individually, andsubsequently-combine the modified detected signals to reproduce, asclosely as possible, the originally transmitted signal.
 26. The systemof claim 17, further comprising a network for performing digital signalprocessing on a plurality of detected signals, individually, andsubsequently combining the modified detected signals to reproduce, asclosely as possible, the originally transmitted signal.
 27. The systemof claim 17, wherein the logic is further configured to provide forwarderror correction for coding formats that include at least one of blockcodes, Reed-Solomon codes, product codes, turbo product codes, turbocodes, low density parity-check codes, and convolutional codes.
 28. Thesystem of claim 17, wherein the logic is further configured to determinethe reliability of row symbols and column symbols for a decoded row anda decoded column based in part on real number valued informationreceived from the resultant output signal, wherein the logic is furtherconfigured to pass the reliability determinations of the row symbols todecode a next column of symbols and pass the reliability determinationsof the column symbols to decode a next row of symbols.
 29. The system ofclaim 17, wherein the logic is included in at least one of a discretelogic circuit having logic gates for implementing logic functions upondata signals, an application specific integrated circuit havingcombinational logic gates, a programmable gate array, and a fieldprogrammable gate array.
 30. The system of claim 17, wherein the logicis included in a computer-readable medium.
 31. The system of claim 17,further including at least one of a processor, memory, and a thresholddevice that communicates with the logic in providing decodingfunctionality.
 32. The system of claim 31, wherein the processor and thelogic are located in separate devices.
 33. The system of claim 31,wherein the processor and the logic are located in the same device. 34.A system for correcting modal dispersion and errors in an optical fibersystem, comprising: means for individually detecting a plurality ofmodes exiting an optical fiber; means for correcting for timingdifferences in the plurality of modes; means for converting opticalsignals from the plurality of modes into an electrical output; means formodifying the electrical output to minimize effects of modal dispersion;and means for implementing forward error correcting on the modifiedelectrical output.
 35. A system for decoding information transferredover a multimode fiber link, said system comprising: logic configured todetermine the reliability of symbols received in a signal output from aphotodetection system, wherein the reliability of the symbols is basedin part on real number valued information received from the symbols,wherein the logic is further configured to decode the symbols using thereliability determinations.
 36. A method for detecting and correctingfor modal dispersion and decoding in a multi-mode fiber optic systemhaving an optical signal coupled into a multi-mode fiber, comprising thesteps of: detecting a plurality of optical signals radiating from an endof the multi-mode fiber by a multisegment photodetector having differentdetector regions that detect different portions of the plurality ofoptical signals; modifying detected signals by the multisegmentphotodetector to reduce effects of modal dispersion among the pluralityof optical signals; and forward error correcting the modified detectedsignals.
 37. The method of claim 36, wherein the step of modifyingdetected signals by the multisegment photodetector includes usingweighting factors that adjust the detected signals.
 38. The method ofclaim 36, further comprising the steps of converting the plurality ofoptical signals into at least two electrical signals and modifying atleast one signal by weighting factors to produce a signal thatapproximates a value of the optical signal originally coupled into themulti-mode fiber.
 39. The method of claim 38, further comprising thestep of combining at least two signals modified together to produce themodified detected signals.
 40. The method of claim 36, wherein the stepof modifying detected signals by the multisegment photodetector includesaltering bias among the multiple detection regions as the weightingfactor.
 41. The method of claim 36, wherein the step of modifyingdetected signals by the multisegment photodetector includes usingattenuation as the weighting factor.
 42. The method of claim 36, whereinthe step of modifying detected signals by the multisegment photodetectorincludes using amplification as the weighting factor.
 43. The method ofclaim 36, wherein the step of modifying detected signals by themultisegment photodetector includes using phase shifting as theweighting factor.
 44. The method of claim 36, wherein the step ofmodifying detected signals by the multisegment photodetector includesusing delay as the weighting factor.
 45. The method of claim 36, whereinthe step of modifying detected signals by the multisegment photodetectorincludes arbitrarily selecting the weighting factors.
 46. The method ofclaim 36, wherein the step of modifying detected signals by themultisegment photodetector further comprises examining an output of themulti-segment photodetector and adjusting a weighting factor until theoutput approximates a value of the optical signal.
 47. The method ofclaim 36, wherein the step of detecting a plurality of optical signalsradiating from an end of the multi-mode fiber by a multi-segmentphotodetector includes using the multisegment photodetector having atleast two concentric, coplanar, and annular photodetectors.
 48. Themethod of claim 36, wherein the step of detecting a plurality of opticalsignals radiating from an end of the multi-mode fiber further comprisesinserting a diffractive optical element between the fiber and themultisegment photodetector for modifying the distribution of opticalsignals among the plurality of detection regions.
 49. The method ofclaim 36, wherein the step of detecting a plurality of optical signalsradiating from an end of the multi-mode fiber further comprisesinserting a reflective optical elements between the fiber and themultisegment photodetector to modify the distribution of optical signalsamong the plurality of detection regions.
 50. The method of claim 36,wherein the step of forward error correcting includes forward errorcorrecting symbols formatted in at least one of block codes,Reed-Solomon codes, product codes, turbo product codes, turbo codes, lowdensity parity-check codes, and convolutional codes.
 51. The method ofclaim 36, wherein the step of forward error correcting includes thesteps of determining the reliability of row symbols and column symbolsfor a decoded row and a decoded column based in part on real numbervalued information received from the resultant output signal and passingthe reliability determinations of the row symbols to decode a nextcolumn of symbols and passing the reliability determinations of thecolumn symbols to decode a next row of symbols.
 52. A method fordetection and compensation of multimodes produced from a multimodeoptical fiber system and decoding, comprising the steps of: convertingan input electrical signal to an optical signal; launching an opticalsignal into a multimode fiber; positioning a photodetection system at anend of the multimode fiber to receive a plurality of optical signalsexiting the multimode fiber; detecting the multiple optical signals bymultiple detectors of the photodetection system producing detectedelectrical signals; modifying the detected electrical signals; addingtogether the detected electrical signals to generate an outputelectrical signal corresponding to the input electrical signal; andforward error correcting the output electrical signal.
 53. The method ofclaim 52, further comprising the step of inserting an interveningoptical element between the fiber and photodetection system to alter thedistribution optical light to the plurality of detection zones.
 54. Themethod of claim 52, wherein the step of modifying the detected opticalsignals further comprises the step of introducing a delay to any of thedetected optical signals.
 55. The method of claim 52, wherein the stepof modifying the detected optical signals further comprises the step ofattenuating any of the detected optical signals.
 56. The method of claim52, wherein the step of modifying the detected optical signals furthercomprises the step of biasing any of the detected optical signals. 57.The method of claim 52, wherein the step of modifying the detectedoptical signals further comprises the step of amplifying any of thedetected optical signals.
 58. The method of claim 52, wherein the stepof modifying the detected optical signals further comprises the step ofphase shifting any of the detected optical signals.
 59. The method ofclaim 52, wherein the step of modifying the detected optical signalsincludes using instruments including at least one of electronic,semiconductor and mechanically based instruments.
 60. The method ofclaim 52, wherein the step of forward error correcting includes forwarderror correcting symbols formatted in at least one of block codes,Reed-Solomon codes, product codes, turbo product codes, turbo codes, lowdensity parity-check codes, and convolutional codes.
 61. The method ofclaim 52, wherein the step of forward error correcting includes thesteps of determining the reliability of row symbols and column symbolsfor a decoded row and a decoded column based in part on real numbervalued information received from the resultant output signal and passingthe reliability determinations of the row symbols to decode a nextcolumn of symbols and passing the reliability determinations of thecolumn symbols to decode a next row of symbols.
 62. A method fordecoding information transferred over a multimode fiber link, saidmethod comprising the steps of: determining the reliability of symbolsreceived in a signal output from a photodetection system, wherein thereliability of the symbols is based in part on real number valuedinformation received from the symbols; and decoding the symbols usingthe reliability determinations.
 63. The method of claim 62, wherein thesteps of determining and decoding includes the steps of determining thereliability of row symbols and column symbols for a decoded row and adecoded column based in part on real number valued information receivedfrom the resultant output signal and passing the reliabilitydeterminations of the row symbols to decode a next column of symbols andpassing the reliability determinations of the column symbols to decode anext row of symbols.